A conventional medical imaging equipment can only offer a two-dimensional image of the inside human organs. A doctor can empirically estimate the size and shape of nidus from a plurality of 2D images so as to “conceive” a three-dimensional geometrical relationship between the nidus and its surrounding tissues, which brings difficulty to therapy. A 3D visual technology can reconstruct a 3D image from a series of 2D images and display it on a terminal. Thereby, not only a profile of the imaged object is obtainable directly and vividly, but also a lot of important 3D information is saved. Since the ultrasound imaging has significant advantages over CT, MRI in non-invasion, non-ionizing radiation and operating flexibility etc., the 3D ultrasound imaging has the prospect of being widely in clinic. There exists a need to expand the research on the 3D visual technology in the ultrasound field.
So far, there are two ways to acquire 3D ultrasound volume data: one is to acquire a series of 2D ultrasound images for a tissue with known spatial positions by using an existing 2D ultrasound diagnostic equipment in combination with a locating mechanism so as to finally obtain the 3D volume data in an offline manner; the other is to utilize a 2D matrix array probe to emit ultrasound beams in a pyramid volume so as to acquire real-time 3D volume data.
There have been some mature methods for visualizing 3D volume data. However, speckle noise specific to the ultrasound image has become an important factor influencing the imaging effect, thus limiting its application. The 3D ultrasound volume data consists of a series of ultrasound images. In forming the ultrasound image, reflection echoes from the reflection points whose space is smaller than a wavelength interfere with each other so as to produce the speckle noise, which will blur the image features and disturb observation and analysis. If such a volume data not being processed is used in 3D imaging directly, the resulting visual effect will be unacceptable. The purpose of preprocessing is to filter or smooth the ultrasound volume data to suppress the noise, so that the visualization after preprocessing leads to imaging of good quality, therefore putting 3D ultrasound imaging to the best use.
A filter to reduce speckle artifact in ultrasound imaging is provided in a patent document US005409007A. In this document, a method for preprocessing is described as comprising the following steps: forming for each pixel in an ultrasound image (prior to a scanning conversion, that is, before a digital scan converter) a five-point diamond-shaped template centering at the pixel; calculating a medium gray-scale value of these five pixels; and replacing the original gray-scale value of the central pixel with the medium value, as shown in FIGS. 1 and 2.
The shortcoming of the above-mentioned technology is that it can not be used in 3D application directly. Even though it is extended to 3D application and the size of the template is adjusted, either the noise is significant or the imaging result becomes blurry, unable to smooth the imaging data without compromising important details. Therefore, the prior techniques are not adapted for practical application.